Digital imaging devices (or imaging devices) such as digital cameras and other electronic image capture devices capture images of visual scenes (or scenes) using image sensors such as charge-coupled device (“CCD”) image sensors or complementary metal-oxide semiconductor (“CMOS”) image sensors that include a number of sensor elements commonly known as pixels. Each pixel collects light from the scene that is to be captured. Typically, each pixel collects optical energy (i.e., light) corresponding to a particular color (e.g., red, green, or blue) that is directed to that pixel by a lens system of the imaging device. Some imaging devices include a dedicated image sensor for each color of light that is captured by the imaging devices. Other imaging devices include a single image sensor rather than a dedicated image sensor for each captured color of light. In such imaging devices, the light that is transmitted to the image sensor is filtered so that each individual pixel collects light from a single color. This filtering is typically achieved using a two-dimensional color filter array that is overlaid on image sensor.
Many color filter arrays comprise a mosaic of color filters that are aligned with the pixels of the image sensor. One filter array is based on a Bayer pattern. When a Bayer pattern is used, filtering is provided such that every other pixel collects green light and pixels of alternating rows (or columns) collect red light and blue light respectively, in an alternating fashion with pixels that collect the green light.
Lens systems used in imaging devices typically include a number of lens elements and an IR-cut filter that acts as an optical bandpass filter. For example, a lens element can be aligned with each pixel to direct or focus optical energy (or light) to that pixel. Due to manufacturing limitations, the lens power, the IR-cut filter, and the collection angle of the lens element aligned with each pixel, the light intensity and frequency (i.e., color) collected at each pixel to vary spatially across the image sensor.
Image sensors are typically calibrated as part of the manufacturing process. For example, raw images from an image sensor are captured under simulated illuminants (i.e., light sources that accurately simulate different illuminants) during manufacturing and a correction mask is generated for each illuminant during a calibration process. The correction mask for a given illuminant is then applied to images captured under that illuminant to correct the images for variations in, for example, the spectral response of the image sensor to the emission spectra of the different illuminants.